This invention relates to the field of charged particle beam lithography systems, and more particularly to a method for fabricating a lens array for use with a multiple charged particle beam exposure system.
As integrated circuits become more complex there is a general trend in the semiconductor industry toward increasing device packaging density. In this way the number of circuit elements per die can be increased, or the die size can be reduced. The present philosophy is to make chip or die sizes as small as possible, as this increases the device yield. However, it is obvious that chip size cannot be arbitrarily reduced because of the inherent resolution limits of presently used photolithographic processes. In particular, the wavelength of light imposes a barrier to the reproduction of detail in the region of one .mu.m(micrometer).
a number of solutions have been proposed to solve the resolution problem and there is considerable activity in this area presently underway in the semiconductor industry. These solutions are based on lithographic techniques that employ particles of shorter wavelengths than visible light to overcome the resolution limitation. Generally, two classes of short wavelength particles have been proposed as alternatives to light, i.e., high energy photons (x-rays) and electrons.
An optimal lithographic system would have to possess certain attributes in order for it to be a serious contender for use in the production of integrated circuits. These attributes include resolution, coverage, lithographic speed, reregistration capability and stability.
At present, minimum reproducible line widths on the order of 1 .mu.m are in use, and future integrated circuit structures will probably push the desired system resolution requirements to below 1/4 .mu.m. The exposure system should be capable of covering the standard 75 mm and/or 100 mm wafers presently used in industry and be expandable to cover the larger sized wafers (125-150 mm) being contemplated. Clearly wafer exposure times measured in tens of minutes to hours are not acceptable in production exposure systems. Acceptable throughput conditions demand full 75 mm and/or 100 mm wafer exposure times of at most a few minutes.
A parallel charged particle beam system is described in a commonly assigned copending application of Eugene R. Westerberg and Ivor Brodie, Ser. No. 227,620, filed Jan. 23, 1981, the disclosure of which is incorporated herein by reference. In this system, the throughput capability of conventional types of lithography systems is increased significantly by utilizing a multiple charged particle beam exposure system for directly writing an integrated circuit pattern simultaneously at a plurality of locations on a target surface. In one embodiment of the disclosed system, an electron source produces an electron beam which is used to illuminate an object aperture. A screen lens consisting of a multiplicity of holes breaks up the flood electron beam emanating from the object aperture into a multiplicity of beams and focuses them in parallel on a resist-coated substrate. Each hole in the lens plate acts as a small aperture lens when a positive potential is applied to the wafer with respect to the screen lens.
An optimal screen lens array fabrication method would have to be capable of producing coaxial beam limiting apertures and aperture lenses in arrays having certain performance requirements for it to be a serious contender for use in the production of screen lens arrays. In particular, the resulting screen lens arrays should be comprised of two thin plates of conducting material with arrays of holes concentric to better than 25 .mu.m in both planes. Each upper hole or "aperture" should have a diameter in the range of 0.25 to 1.0 mm, and each lower hole or "lens" should have a diameter of 1.0 to 2.0 mm. The spacing between the two planes containing these holes should lie in the range 1.0 to 3.0 mm. The hole diameters given above should be accurate to .+-.25 .mu.m (lens plate-to-lens plate) and to .+-.12 .mu.m within a given plate. Also, the deviation in the diameter of any hole due to ellipticity must be less than 2 .mu.m. Edge burrs should be avoided; but, when present, should not exceed 2 .mu.m if burr projects towards the lens axis and 5 .mu.m when projecting away from the lens axis.
In some applications, the lenses may be constructed from a solid piece of material to simulate the aperture and lens holes.
For standard 100 mm wafers the lens plate should be 100 mm or larger in diameter and contain a minimum of 50 lenslets and a maximum of 1,000 lenslets arranged on a cartesian grid. The spacing between nearest neighbors should be accurate to .+-.25 .mu.m. The aperature plate and the lens plate should be coplanar to within .+-.12 .mu..
Fabrication methods for making screen lens arrays are known in the art. However, the known methods suffer from one or more limitations or drawbacks which preclude their application for making screen lens arrays of the type utilized in the aforementioned patent application of Westerberg and Brodie. For example, although the prior art discloses the utilization of Boron indiffusion techniques, they do not provide satisfactory solutions to the problems which applicants' invention addresses. In particular, the known "spin coating" technique for application of the Boron does not provide a satisfactory screen lens array having the aforementioned performance characteristics. Moreover, the disclosed Boron penetration depths (on the order of 2 .mu.m) do not provide sufficient mechanical strength. And finally, the teachings of the prior art are silent as to what constitutes a workable Boron atom concentration.
It is accordingly a general object of the present invention to overcome the aforementioned limitations and drawbacks associated with known methods and to fulfill the needs mentioned by providing a method for fabricating screen lens arrays for use in a charged particle beam exposure system having all of the desirable attributes noted above.
It is a particular object of the invention to provide fabrication methods for producing high resolution screen lens arrays.
Other objects will be apparent in the following detailed description and the practice of the invention.